Dye-sensitized solar cells (DSCs) are promising photovoltaic devices due to high photoelectric conversion efficiency (η), low manufacturing cost, and potential for manufacturing transparent devices. In a typical dye-sensitized solar cell, a liquid electrolyte (LE) is used as a hole-transport medium. However, leakage and evaporation of the liquid solvent occurs, so that the device efficiency is decreased over time. In order to overcome these problems, a lot of research has suggested substitution of the liquid electrolyte by solid or quasi-solid electrolytes including polymers, p-type semiconductors, hole-transport polymers, and solvent-free ionic liquids. Among these materials, the polymer-based solid electrolytes are competitive in terms of cost and processing. The hole-transport materials have not yet been commercialized. The polymer-based electrolytes are very suitable for manufacturing flexible devices. Various polymers including polyethylene oxide (PEO), poly(vinylidene fluoride-co-hexafluoropropylene), poly(methyl methacrylate) (PMMA), poly(acrylonitrile-co-vinyl acetate), and polyacrylonitrile have been applied to the dye-sensitized solar cells. In a polymer gel electrolyte (PEG), a polymer fixes solvent molecules with van der Waals forces and thus dramatically reduces the amount of usable solvent. The polymer gel electrolyte is often combined with nanoparticle fillers including SiO2, TiO2, and various carbon nanomaterials [e.g., carbon nanotube (CNT), graphene]. The nanoparticle fillers compensate low diffusibility of electrolyte ions and low conductivity of the polymer gel electrolyte film in the polymer matrix. To be more specific, a carbon nanomaterial tends to improve conductivity and dissociation due to its high adsorption of lithium ions. For example, a PEO/CNT complex showed ion diffusibility three times higher than PEO and a PMMA/CNT complex showed conductivity three times higher than bare PMMA.
However, the above-described application of the polymer gel electrolyte to the dye-sensitized solar cell produces some practical problems. When a typical dye-sensitized solar cell is prepared, the electrolytic solution is injected into a gap between a photoelectrode and a counter electrode during a final step of cell assembly. The gap generally has a width of ten of micrometers. Therefore, the injection of the polymer gel electrolyte with a high viscosity requires a high pressure which may affect the cell. Completely injecting the polymer gel electrolyte between mesoscale pores in a typical TiO2 photo-absorption electrode film is also challenging. In this case, most of research on a polymer gel electrolyte in a dye-sensitized solar cell has used very low wt % of a polymer in order to produce a diluted polymer gel electrolyte. There has been an attempt to control a gelation rate of a polymer-dissolved liquid electrolyte in order to facilitate injection of a polymer gel electrolyte. Recently, a novel strategy based on in-situ gelation using a polymer/particle-deposited substrate has been suggested. In this approach, a polymer gel electrolyte was prepared by depositing polymer particles on the electrode, assembling the cell, and injecting an electrolyte solution to dissolve the polymer particles. After the cell was assembled, the polymer gel electrolyte was prepared. Thus, the above-described problems associated with injection and filling of ex-situ prepared polymer gel electrolyte were eliminated. However, the previous approach was verified using only polystyrene particles which have a relatively low compatibility with an acetonitrile-based electrolyte solvent. Therefore, in the in-situ approach suggested above, the use of polymer particles with a high compatibility is still challenging.
Korean Patent Laid-open Publication No. 10-2013-0145664 relates to a composite polymer electrolyte for rechargable lithium battery and a manufacturing method thereof, and discloses a polymer electrolyte in a solid state using a branched polymer and graphene oxide.